The invention relates to a mandrel-holder for a pilger rolling mill, the holder being in one piece and comprising a head defining a seat adapted to receive and hold a tang end of a mandrel, and a body adapted for coupling to an actuator used for driving the mandrel-holder in rotation and in a straight line.
Usually the mandrel-holder head has a substantially U cross-section and the mandrel is secured to the mandrel-holder by inserting the mandrel tang between the sides of the head and removably securing it, e.g. by substantially male-female means. A bush is also used for coupling and is coaxially fitted on to the mandrel near its tang and adapted to engage a corresponding, substantially semiannular seat formed in the free end of the mandrel-holder head.
To ensure that the pilger rolling mill operates properly, it is necessary to prevent the mandrel coming loose from the mandrel-holder, as may happen, particularly in the case of older mandrel-holders, if the holder is in the upturned U position and the bush is not securely fitted in the corresponding seat of the mandrel-holder.
It has been frequently found that the aforementioned prior-art devices do not guarantee proper operation of the machine, more particularly when there is an increase in the speed of rolling and the angle of rotation during the positioning phase, e.g. in the case of more modern rolling mills, where use is also made of special devices which, even in the positioning stage, cause the mandrel and the bored blank to move along the entire stroke of the corresponding actuator.
The aforementioned problem has hitherto been solved by using devices which are mounted on the mandrel-holder and can move and secure an abutment means which is stationary in an operative position, in which it engages the mandrel tang (when the rolling mill is in operation) whereas when the abutment is in the inoperative position it is retracted into the mandrel-holder and disengaged from the tang (when the rolling-mill stops). The aforementioned devices usually comprise a double-acting pneumatic cylinder borne by the mandrel-holder with its axis parallel to the rolling axis and having a suitably dimensioned rod which constitutes the stationary abutment means for the mandrel tang.
It is very difficult, and not always possible, to ensure appropriate pneumatic communication between the ducts supplying the cylinder, which moves integrally with the mandrel holder, and the ducts from a compressed air source which is stationary and positioned outside the rolling mill. It is conventional to use stationary air chambers which partially surround the mandrel holder, but they are frequently inadequate and also constitute obstacles which undesirably reduce the accessibility of the mandrel-holder.
Furthermore, if the aforementioned devices are used, there is a need for suitable valve means for cutting off the compressed air, and for gaskets ensuring perfect sealing-tightness to compressed air under rather critical operating conditions. Another technical, more particularly economic, disadvantage is the possibility that the stationary abutment, i.e. the cylinder rod used for this purpose, will not be completely disengaged from the mandrel tang at the end of the rolling operation, so that when the mandrel is released from the mandrel-holder the abutment may be sheared away or sufficiently deformed to become unserviceable.
The invention is based on the problem of constructing a mandrel-holder for a pilger rolling mill having structural and functional characteristics which guarantee optimum engagement between the mandrel and the mandrel-holder during any phase of operation of the rolling mill, thus simultaneously overcoming the disadvantages mentioned hereinbefore with reference to the prior art.